The present invention relates to a circular knitting machine for socks, stockings or the like, in particular for producing knitting with towelling stitches.
As known, in circular knitting machines loop pile stitches are formed by using casting-off sinkers which have two casting-off planes arranged at mutually different vertical levels and by feeding the needles with two threads at points arranged at mutually different vertical levels.
Upstream of the thread feed along the direction of rotation of the needle cylinder with respect to the needle actuation cams, the sinkers are moved, by means of actuation cams, toward the outside of the needle cylinder so as to release the loops formed by the needles and move the casting-off planes into the position for receiving the thread which is engaged by the needles. The subsequent lowering of the needles between the sinkers forms stitches, each of which is composed of two loops with mutually different lengths, i.e. so-called loop pile stitches.
In some situations, the stitches formed by the needles may be defective, since the two threads which compose the loop pile stitch do not rest correctly on the related casting-off plane but instead end on the same casting-off plane of the sinkers, causing the forming of two loops with identical lengths.
Knitting defects of this type occur more frequently in circular machines for very finely knitted socks or stockings during the knitting of the tip or heel thereof.
As known, when the knitting of the heel or tip of the sock or stocking begins, one half of the needles of the machine is in fact excluded from knitting, whereas the other half of the needles continue to form knitting at one feed of the machine.
The needles excluded from knitting are raised with respect to the other needles and are kept in this position until the heel or tip is completed.
The needle cylinder is actuated with an alternating rotary motion about the needle cylinder axis and at every forward or reverse stroke of the needle cylinder the first needle of the set of needles which has just knit at the feed being considered is raised and excluded from knitting in order to achieve the progressive casting off of stitches typical of the forming of the heel or tip. The needles excluded from knitting keep the last formed loop on their stem.
Knitting defects in the forming of loop pile stitches in this step can be mainly ascribed to the traction exerted on the fed threads by the loops carried by the needles excluded from knitting. During the forming of the heel or of the tip, the threads fed to the feed being considered are engaged with the last needle of the set of needles which does not knit at said feed and which is therefore raised with respect to the subsequent needle, which instead must engage the threads supplied by said feed. For this reason, both of the fed threads are subjected to an upward traction which prevents the thread fed at a lower level from resting on the lower casting-off plane of the sinkers and instead makes it rest on its upper casting-off plane together with the other thread. This traction is also increased due to the cast-offs, since the last needle of the set which does not knit at the feed being considered has been excluded from the knitting, and has thus been raised, directly upstream of said feed.
The upward traction effect, the force of which increases as the rigidity of the threads increases and as the fineness of the machine becomes greater, easily affects the entire row of knitting being formed, producing a defective row which can lead to the rejection of the manufactured sock or stocking.